The present section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present disclosure that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the following, a picture contains one or several arrays of samples (pixel values) in a specific picture/video format which specifies all information relative to the pixel values of a picture (or a video) and all information which may be used by a display and/or any other device to visualize and/or decode a picture (or video) for example. A picture comprises at least one component, in the shape of a first array of samples, usually a luma (or luminance) component, and, possibly, at least one other component, in the shape of at least one other array of samples, usually a color component. Or, equivalently, the same information may also be represented by a set of arrays of color samples, such as the traditional tri-chromatic RGB representation.
A pixel value is represented by a vector of n values, where n is the number of components. Each value of a vector is expressed in a number of bits which defines a maximal dynamic range of the pixel values.
A picture block means a set of pixels which belong to this picture and the pixel values of a block mean the values of the pixels which belong to this block. A picture block may be a macroblock, a Coding Unit or a Transforming Unit according to H264 or H265, or any other set of pixels forming a region of picture of any arbitrary shape (not limited to a square or rectangular shape).
Low-Dynamic-Range pictures (LDR pictures) are pictures whose luminance values are expressed in a limited number of bits (most often 8 or 10). This limited expression does not allow correct rendering of small signal variations, in particular in dark and bright luminance ranges. In high-dynamic range images (HDR images), the signal expression is extended in order to maintain a high accuracy of the signal over its entire range. In HDR images, pixel values are usually expressed in floating-point format (either 32-bit or 16-bit for each component, namely float or half-float), the most popular format being openEXR half-float format (16-bit per RGB component, i.e. 48 bits per pixel) or in integers with a long expression, typically at least 16 bits.
A typical approach for encoding an HDR image is to apply a transfer function to the picture in order to reduce the dynamic range of the picture (typically 8 bits). Then, the transferred picture is encoded by means of a traditional encoding scheme (JPEG/JPEG200, JPEG2000: “Information technology—JPEG2000 image coding system: Code coding system”, ITU-T Recommendation T.800, August 2002, or MPEG-2, H.264/AVC, H.264/AVC: “Advanced video coding for generic audiovisual services”, Recommendation ITU-T H.264 (Edition 9), February 2014 or H265/HEVC, HEVC: “High Efficiency video coding”, Recommendation ITU-T H.265, October 2014 for video.
A typical transfer function is defined in a contribution to the ITU standard organization entitled “Image dynamic range in television systems”, document 6C/77-E, 4 Oct. 2012”.
At the decoder side, a transferred picture is obtained and an inverse transfer function is applied on the decoded transferred picture. The term “inverse transfer function” means that if a transfer function and an inverse transfer function are applied successively to an input picture (without any encoding/decoding), the resulting picture and the input picture are the same picture. Any monotone function may be a transfer function.
Using such a typical encoding scheme for encoding an HDR picture involves large sizes of the compressed pictures. Consequently, such typical approaches cannot be used in applications requiring high coding performances such, for example, in a transmission context where high compression rates are required.
The present disclosure has been devised with the foregoing in mind.